fix libzstd-mt underlinking issue

fix #2045
When compiling `libzstd` in multithreading mode,
the `libzstd-mt` recipe would not include `-pthread`,
resulting in an underlinked dynamic library.

Added a test on Travis to check that the library is fully linked.

This makes it possible, in some future release,
to build a multi-threaded `libzstd` dynamic library by default
as it would no longer impact the build script of user programs.

lib/Makefile

@@ -154,9 +154,6 @@ libzstd.a: $(ZSTD_OBJ)
 	@echo compiling static library
 	@$(AR) $(ARFLAGS) $@ $^
 
-libzstd.a-mt: CPPFLAGS += -DZSTD_MULTITHREAD
-libzstd.a-mt: libzstd.a
-
 ifneq (,$(filter Windows%,$(OS)))
 
 LIBZSTD = dll\libzstd.dll
@@ -180,17 +177,20 @@ endif
 
 libzstd : $(LIBZSTD)
 
-libzstd-mt : CPPFLAGS += -DZSTD_MULTITHREAD
+lib : libzstd.a libzstd
+
+%-mt : CPPFLAGS += -DZSTD_MULTITHREAD
+%-mt : LDFLAGS  += -pthread
+
 libzstd-mt : libzstd
 
-lib: libzstd.a libzstd
+libzstd.a-mt: libzstd.a
 
-lib-mt: CPPFLAGS += -DZSTD_MULTITHREAD
-lib-mt: lib
+lib-mt : lib
 
-lib-release lib-release-mt: DEBUGFLAGS :=
+%-release : DEBUGFLAGS :=
 lib-release: lib
-lib-release-mt: lib-mt
+lib-mt-release : lib-mt
 
 # Special case : building library in single-thread mode _and_ without zstdmt_compress.c
 ZSTDMT_FILES = compress/zstdmt_compress.c